1. Field of the Invention
The present invention relates to optical sensing and, more particularly, to an optical sensor which utilises the phenomenon of surface plasmon resonance.
2. Discussion of the Background
Surface plasmons are the quanta of a surface electromagnetic wave propagating along the interface between a conducting material such as a metal and a dielectric. They represent the coupling of a longitudinal oscillation of the surface charge density with its associated electromagnetic fields. Surface plasmons can be excited by an electric field of light if the component of the wave vector of the light wave along the surface equals that of the same frequency surface wave. The effect is then called surface plasmon resonance (SPR) and may be seen as an absorption of the light. Kretschmann and Raether (Z. Naturforsch., (1968) 23a, 2135) have produced a comprehensive description of the phenomenon. The effect is strongly dependent on the dielectric properties at the metal surface and can therefore be used to sense materials at or deposited on the metal surface or to sense changes brought about in a previously existing overlayer by exposure to some other substance.
Some known forms of optical sensors utilise this phenomenon by scanning the angle of incidence or by using the so called convergent beam method. In the latter approach polarised monochromatic or narrow band light is first expanded and then focused on to a metal film coated onto the back surface of a glass prism or the front surface of a diffraction grating. The position of resonance is detected by using a linear diode array and the source of radiation or light is usually a light emitting diode which requires polarisation filters for use as an SPR source or a polarised gas laser such as a helium-neon laser.